I remember horror stories about the 556R(7.62x39) but nothing about the
5.56MM SIG-556.

It IS a great value; you can find the bare bones version for about $1,000.00 and the version with the quad rail for about $1,100.00.
Most AK's for that price will not have quite the same fit for finish. Most Piston AR's are $200+ above that price point.

If you understand the fact that the 556 IS NOT an AR and it is NOT an AK and you may not be able to change every little thing about it like an AK or an AR then it will be good. If you are simply looking to turn a SIG556 into a Piston AR/AK hybrid better to just buy a Piston AR for the extra $$$$.

If you are really interested in the SIG556 then check out the SIG556 forum; it has a wealth of info about the rifle.http://www.sigarms556.com/

DORC-1 is right though; perfect example is the SIG M400. You could find them for $800.00 last summer before they really caught on. Now the exact same rifle is about $1,000.00+ on average. Went from one of the better value for the money AR's out there to overpriced.